1. Field of the Invention
The present invention relates generally to circuit boards, and, more particularly, to an apparatus and method for obtaining a thin, flexible stripline flex cable with shifted, symmetrical, cross-hatched reference planes.
2. Description of the Prior Art
Microstrips are extensively used to form flexible cables to interconnect high-speed logic circuits in digital computers because they can be fabricated by automated techniques and can provide the required impedance-controlled signal paths. However, microstrip construction permits significant levels of extraneous electromagnetic radiation. Stripline construction can be utilized to greatly reduce unwanted electromagnetic radiation. However, the addition of a second reference or ground layer results in increased capacitive coupling between the signal conductor and the reference planes, thus greatly reducing the impedance of the signal conductor. In order to maintain the desired impedance, the distance between reference layers and the signal conductor in traditional stripline construction must be greater than in microstrip construction. This increased thickness significantly reduces the cables' ability to withstand multiple flexures.
Typically conductors are formed in planes in the flexible cable. Ground planes or other reference voltage planes are positioned in planes parallel to the conductor planes to control the impedance of the conductors and to block the transmission of electromagnetic radiation from conductors carrying high frequency signals, such as clock signals and high speed data signals found in digital computers. In printed circuit boards, and the like, solid ground planes are generally used. However, solid ground planes are inflexible unless made very thin, and thus cannot be readily used to protect signal lines in cables that are intended to be frequently flexed, for instance, in the signal lines between the base and the movable display screen of a notebook computer. Furthermore, because of the large capacitance of a solid ground plane formed close to a signal line, the impedance of the signal lines may be lower than desired. On the other hand, if the ground plane is spaced further apart from the conductors to reduce the capacitance and thereby increase the impedance, the flexible cable becomes thicker and thus less flexible and more likely to break with repeated flexing.
Reference planes having conductive elements formed in a grid have been utilized in microstrip designs to increase the impedance and to provide flexibility. However, because the grid is not continuous like a solid reference plane, it has been found to be quite difficult to control the impedance of signal lines protected by a gridded reference plane because of the uncontrolled orientation of the signal lines with respect to the shielding grid. It is thus a goal of the present invention to provide a circuit board that provides effective shielding and having a controllable characteristic impedance.
One of the particular difficulties in controlling the impedance characteristics of flexible cables utilizing cross-hatched reference planes, especially cables of stripline construction, has been in the structure of turns in the cable. For example, when a flexible cable has a branch of 90 degrees, or the like, the orientation of the signal lines with respect to the shielding grid varies as the signal lines change direction, thus causing significant changes in the impedance of the signal lines. Thus, another goal of the present invention is to provide a means of reorienting signal lines which minimizes impedance discontinuities.
Accordingly, there is a need in the art to provide a circuit board having flexible reference planes, capable of thousands of flexures in a stripline design, and achieving a desired impedance that permits transfer of the signals without degradation to the signal quality while providing an acceptable shielding capability.